This invention relates to a network node, and more particularly, to a technology of controlling a transmission timing for a packet to be transferred.
The recent broadening, speed-up, and cost reduction of communication networks expand the fields to which the network technology is applied. This increases demands for achieving more reliable communication.
While there are many factors in the communication reliability, one of the reliabilities in communication of best effort type such as the existing Ethernet (trademark) that are difficult to achieve is to guarantee packet inter-arrival interval jitter. In a network node, a buffer is provided with a queue that stores packets to be received from a plurality of lines, and packets to be transferred are processed in order. Accordingly, the processing time needed at a network node varies depending on the existence/absence of traffic, i.e., the difference in number of packets stored in a queue. The time delay caused by the time period during which packets are stored in the queue of a network node in this way is the queuing delay.
One method of improving the communication reliability is to suppress generation of jitters caused by a variation in queuing delay in a transmission path. For example, suppressing delay-originated jitters brings about an effect of improving the quality of real-time communication, such as voice calling and video streaming, an effect of reducing the capacity of the buffer in a terminal, and other such effects.
JP 2002-271388 A discloses a technology designed to achieve such effects. Specifically, according to the technology disclosed in JP 2002-271388 A, an apparatus on the transmission side adds time information on a reproduction timing to a real-time packet, and an apparatus on the reception side refers to the time information to adjust the timing for a buffered packet and transmits the packet to a terminal. This enables a terminal-side gateway to remove the influence of the queuing delay caused in the transmission path for the real-time packet.
To improve the accuracy in the time synchronization method over a network, it is important to reduce jitters. As the time synchronization method over a network, the Network Time Protocol (NTP) or the like has hitherto been used. In recent years, on the other hand, the smart grid, the base station network for cellular phones, and the like tend to employ IEEE 1588 v2 (PTP: Precision Time Protocol) as a method that enables more accurate time synchronization in a wider area.
In the IEEE 1588 v2, in order to increase the accuracy of time synchronization, an error in the synchronous time caused by the difference in delay time on a transmission path on a network is reduced. Building a network that uses the IEEE 1588 v2, on the other hand, needs a network node that has the function of Boundary Clock or Transparent Clock.
The Boundary Clock has a clock for synchronization inside a network node, and synchronizes the time with a host apparatus by using a synchronous packet received from the host apparatus. Further, the Boundary Clock synchronized with a host apparatus transmits a synchronous packet to a subordinate apparatus to synchronize the clock of the subordinate apparatus.
The Transparent Clock causes a network node to measure the time required for a synchronous packet to pass through the Transparent Clock, and to add a compensation for the time to the synchronous packet. As a result, both apparatus synchronized with each other can ignore the difference in delay time for each packet passing the Transparent Clock.
With such a method, synchronization achieved by the IEEE 1588 v2 secures high time synchronization accuracy. On the other hand, a network node compatible with the Boundary Clock or Transparent Clock is needed to build a network, and hence it is not possible to effectively use the conventional network resources built with the Ethernet or the like. Moreover, the complicated functions of the Boundary Clock and Transparent Clock lead to cost increase and size increase.
In this respect, as a related art, JP 2007-134873 A discloses a technology for improving the synchronization accuracy through the arithmetic operations in an apparatus that receives synchronous packets. In the technology disclosed in JP 2007-134873 A, even when a node constructing a network does not have the functions of the Boundary Clock and Transparent Clock, the synchronization accuracy is secured through arithmetic operations at a terminal node to be synchronized. With the use of transmission of time synchronous packets at a given interval, a terminal node monitors the arrival intervals of time synchronous packets to measure a queue-originated jitter, and correct the synchronous time or estimate the degree of congestion of a network.